o 



62 Of the Propagation of Heat 



in our attempts to contemplate those invisible operations 

 of nature which nothing but the sharpest ken of the in- 

 tellectual eye will ever be able to detect and seize. 



As succeeding events which fall under the cognizance 

 of our senses cannot be distinguished if they happen 

 oftener than about ten times in a second* it appears that 

 when a particle of water moves in a quiescent mass of 

 that fluid at the rate of y-J-^- part of an inch only, in one 

 second, its succeeding collisions with the different parti- 

 cles, at rest, of that fluid, against which it strikes as it 

 moves on, must be so inconceivably rapid that no less 

 than one thousand of them must actually take place, one 

 after the other^ in the shortest space of time that is per- 

 ceptible by the human mind, f 



* This assertion, in as far, at least, as it relates to objects of sight, maybe proved by 

 the following easy experiment : Let a wheel, with any known number of spokes, be 

 turned round its axis with such a velocity as shall be found necessary, in order that the 

 spokes may disappear or become invisible. From the velocity of the wheel, and the 

 number of spokes in it, the fact will be decided. 



\ It probably will not escape the observation of my learned readers, that the velocity 

 which I have here assigned to the single particle of water, moving upwards in that fluid 

 in consequence of a change of its specific gravity by Heat, though apparently 

 very small (y-l-g- part of an inch in a second), is however, most probably, consider- 

 ably greater, in fact, than any individual solitary particle of that fluid could possibly 

 acquire," in the supposed circumstances, by any change of temperature, however great, 

 owing to the resistance which would necessarily be opposed to its motion by the qui- 

 escent particles of the fluid. Aware of this objection, and being desirous of being 

 prepared to meet it, I took some pains to compute, by the rules laid down by Sir 

 Isaac Newton in his Principia, Book II. Sect, vii., what the greatest velocity is that a 

 solitary particle of water (supposed to be ■^jj-q\-(^q-q of an inch in diameter) could pos- 

 sihly acquire by a given change of its specific gravity. And I found that if the spe- 

 cific gravity of water at the temperature of 32° F. be taken at 1.00082, and its speci- 

 fic gravity at 8c° at 0.99759, as lately determined by accurate experiments, then a 

 single particle of water at the temperature of 8g°, situated in a quiescent mass of that 

 fluid at 32°, the greatest velocity this hot particle could acquire in moving upwards in 

 consequence of its comparative levity would be that of oeVs P^'^'' °^ ^^ inch in 1 

 second. This is at the rate of about one inch and an half in i hour. But it is evi- 

 dent, that when great numbers of particles unite and form currents, they will make 

 their way through the quiescent fluid with greater facility, and consequently will 

 move faster. 



